The functions of a beam guiding and beam shaping system in a high-power CO2 laser system are to guide the laser beam as far as possible no loss of power and quality and to shape the beam to produce the desired power density distribution at the processing site. When selecting the optical elements, the particular requirements of the infrared CO2 wavelength and the high power densities are to be taken into consideration. Owing to the long wavelength (>2 μm) of CO2 lasers, beam guiding is carried out predominantly in free beam propagation by means of reflective, transmissive and partially transmissive optical elements and not by means of optical glass fibers since the attenuation losses are too great. For many applications there is the possibility of using both reflective and transmissive and/or partially transmissive optical elements.
At power densities above 1-2 kW/cm2, reflective optical elements are preferably used since they have higher destruction thresholds and undergo less thermal deformation owing to the possibility of more effective cooling over the rear side of the optical elements. Transmissive and partially transmissive optical elements have the disadvantage that cooling may occur only over the edge of the optical elements, whereas in the case of reflective optical elements uniform cooling may occur over the rear side of the optical element. Uneven irradiation of an optical element with a maximum on the optical axis and cooling over the mounting at the edge of the optical element give rise to an extremely inhomogenous thermal load concentrated in the center of the optical element. Compared with transmissive optical elements, reflective optical elements have the disadvantage of greater sensitivity to adjustment. Furthermore, transmissive or partially transmissive optical elements, such as, for example, lenses, beam splitters and output mirrors, are indispensible as closure windows and protection windows of process gas chambers and for beam coupling and decoupling, since alternatives, such as, for example, aerodynamic windows, are comparatively expensive.
At the infrared wavelength of a CO2 laser beam of 10.6 μm, only a few optical materials are available for transmissive optical elements. Zinc selenide (ZnSe) is predominantly used for transmissive optical elements. The advantages with that material are especially the small absorption coefficient and the low dependency of the refractive index on temperature. In comparison with zinc selenide, gallium arsenide (GaAs) has a higher absorption coefficient which, however, is compensated for by better thermal conductivity and mechanical strength. Further optical materials that are used for transmissive optical elements are quartz, BK7 glass and germanium.
Absorption of laser radiation at optical elements is unavoidable and is especially significant for transmissive and partially transmissive optical elements. An incident laser beam is absorbed in the basic material of the optical element, in the coatings and at impurities in or on the optical element and leads to heating of the optical element and alteration of the optical parameters including, but not limited to, refractive index and thermal conductivity. Dust particles or other contaminants, such as, for example, abrasion debris, that are present in a beam guiding chamber may be deposited on the surface of the optical elements and lead to increased absorption of the incident laser beam and hence to additional heating of the optical elements.
Since the beam guiding and beam shaping system has a great influence on the processing result, absorption constitutes a considerable source of error. This may be limited by making high-purity optical materials, by cooling the optical elements and by regular maintenance. Monitoring systems offer the opportunity to discover errors in good time and to reduce the downtimes of optical elements.
The expression “thermal lens effect” is used hereinafter to mean all the effects of absorption on an optical element that result in a change in the optical parameters and in a deterioration of the beam quality. A distinction is to be made between two phenomena: thermal runaway and thermal-optical deformation. Thermal runaway involves an exponential increase in the absorption coefficient with temperature, the consequence of which is that, above a limit temperature, more energy is absorbed than can be dissipated by means of cooling. Continuous heating with increasing absorption would ultimately lead to destruction of the optical element. Since greatly increased absorption is associated with poorer optical quality, the danger of thermal runaway can, in most cases, be recognized in good time and avoided. Thermal-optical deformation is used to mean a geometric deformation of the optical element owing to expansion of the volume as a function of temperature and a change in the refractive index.
The refractive index “n” is a temperature-dependent property of optical elements. Owing to that temperature dependency, a spatially inhomogeneous temperature distribution in an optical element can result in an incident laser beam being refracted to different degrees. With optical materials, a distinction is to be made between materials having a positive refractive index gradient (dn/dT>0) and materials having a negative refractive index gradient (dn/dT<0). In the case of a plane optical element, a temperature distribution having a maximum on the optical axis (e.g., Gaussian temperature distribution) and a positive refractive index gradient result in a focusing of the incident laser beam, whereas a negative refractive index gradient produces a widening of the laser beam. A focusing optical element produces a laser beam flow with a very small beam diameter (beam waist) at the focus of the optical element, and downstream of the beam waist the beam diameter increases. Heating of the optical element leads in the case of a positive refractive index gradient to the beam diameter being reduced in the region of the optical element as far as and just downstream of the beam waist as compared with a cold optical element, whereas the beam diameter becomes larger at intervals that are large relative to the focal length of the optical element as compared with a cold optical element. A negative refractive index gradient leads to the opposite effect, in the case of a heated optical element, i.e. the beam diameter in the region of the optical element as far as and just downstream of the beam waist increases as compared with a cold optical element, whereas the beam diameter decreases at intervals that are large relative to the focal length of the optical element.
Zinc selenide and gallium arsenide have temperature-dependent thermal conductivities and refractive indices and have positive refractive index gradients (dn/dT>0), and this leads to the refractive power of an optical element, and hence the focusing properties, changing with temperature. At high temperatures, the thermal conductivity falls, which leads to steeper temperature gradients with increasing temperature since the heat dissipation becomes poorer. Owing to the positive refractive index gradient for zinc selenide and gallium arsenide, the steeper temperature gradient results in an increased refractive power and hence in an altered propagation of the laser beam downstream of the optical element in comparison with a cold optical element. Partially transmissive decoupling optics, which decouple the laser beam from the laser resonator, and focusing lenses are subject to the highest thermal loads.
EP-A-1 398 612 describes an apparatus for monitoring the functionality of an optical element and is suitable especially for partially transmissive optical elements such as, for example, output mirrors of laser resonators. The monitoring apparatus includes a detector and a light source whose measuring radiation is at least partially reflected by the surface of the optical element facing the detector and the light source. A proportion of the measuring radiation is detected by the detector as a reflected measuring beam. The light source and the detector are diametrically opposite each other with respect to the optical element monitored and are arranged, in particular, at the same angle and to the side of the optical element.
DE-C 198 39 930 describes a method for monitoring the functionality of a transmissive protective element of a laser optical system which is transmissive to the laser wavelength and a device for carrying out that method. The object is to configure a method and a device for monitoring the functionality of a transmissive protective element of a transmissive laser optical system in such a manner that, in particular, crack formation or destruction of the protective element can be reliably detected. The monitoring device includes a detector, which is coupled to the lateral surface of the protective element and which detects light leaving the lateral surface, and a light source which is coupled to the lateral surface and which cooperates with the detector to form a light barrier. The measuring radiation of the light source is coupled into the protective element in a direction at an angle to the direction of incidence of the laser beam and crosses the protective element. After crossing the protective element, a proportion of the measuring radiation is detected by the detector as a transmitted measuring beam. To monitor the temperature of the transmissive protective element, radiation temperature sensors are provided which are intended to monitor the local temperature in different regions of the protective element by detecting the thermal radiation emanating from the protective element. Using different types of radiation temperature sensor it is possible to differentiate slow and rapid temperature changes. However, although the thermal radiation emitted by a transmissive body also depends on parts of the volume in the interior of the optical element, it is to a large extent determined from the surface temperature, which is why determination of the temperature in the interior of the optical element, in particular, is possible only with difficulty on the basis of the data provided by the radiation sensors.